Wire providing microwave/radar frequency attenuation is referred to in the wire and cable trade as "filter line". Some of these cables are referenced by U.S. Mil. Spec. No. Mil-C-85485. The measurement of the attenuation (insertion loss) upon a given wire's performance relates to the effect that the filter line has upon the interference signals which are conducted down the wire.
The ever-increasing high technology requirements of the aerospace industry demand that filter line cables be smaller in size, more flexible, and lighter in weight, while also providing high temperature capabilities. Utilizing the commercial materials that are presently available, some of the latest industry requirements cannot even be met.
This invention seeks to provide new, space-age materials in unique combinations to provide EMI, microwave and radar filtering capabilities in a wider frequency range than heretofore available. At the same time, the invention provides a cable product that utilizes a thinner filtering layer, which reduces the size of the composite cable construction. In addition, the newer materials are lighter; their thinner cross-sections further reduce the weight of the finished cable product. Another advantage of the thinner filtering layer is the increased flexibility of the cable. The high-temperature jacket is also of very thin construction, thus affording high temperature capability without sacrificing the size, weight, and flexibility advantages provided by the thinner inner cable layers.
Recently, ferromagnetic particles of ferrite or magnetite have been coated with metal in order to provide conductive materials having advantageous electrical and magnetic properties. It is contemplated with this invention that these types of materials can be loaded into a polymeric matrix for use as an interference layer in the fabrication of "filter line" cable.
The current invention reflects the discovery that, when ferrites are mixed with certain polymers, they provide easily extrudable compounds that are most suitable for wire and cable fabrication. Such compounds can be directly extruded over bare or insulated wire to form a wire and cable article that attenuates (filters) high frequency interferences down the cable line.
The filter line cable of this invention has been reduced in size. The various layers in the construction have been made exceptionally thin. As a result, the cable is also enhanced by a high-temperature jacket layer. The addition of a thinly overlaid jacket layer provides high temperature capabilities, without sacrificing the advantages provided by the lighter, thinner and more flexible inner layers of the cable construction. This jacket layer comprises a layer or layers of polytetrafluoroethylene; polytetrafluoroethylene and polyimide; or polytetrafluoroethylene, polyimide and polytetrafluoroethylene fused together. Such a configuration will increase the temperature rating of the overall cable up to 260.degree. C., which is a substantial increase over standard "filter line", which has a typical temperature rating of 150.degree. C.
The present invention seeks to fabricate wire and cable articles that provide protection against both of the aforementioned effects, namely, attenuation of signals conducted down the wire and high-temperature effects.
The current invention contemplates a wire or cable construction using a layer composed (at least in part) of silver-coated magnetic particles such as ferrites or magnetites dispersed in a polymeric matrix, such as Viton (a fluorinated elastomeric polymer manufactured by Du Pont). The magnetic particles are provided by various manufacturers, including Steward Manufacturing Company of Tennessee and Fair-Rite Products Corporation of New York. The impedance characteristics of the magnetic particles vary, depending upon the supplier, fabrication conditions and composition. Metal coating (such as silver) is provided by Potters Industries, Inc., of Parsippany, N.J.
High-frequency signals conducted down this wire are partially absorbed by the silver-coated particle shield layer. The electromagnetic waves penetrate through the jacket layer to the filter layer and are partially absorbed by the ferrite particles. They are then dissipated by lattice vibration or phonon emission.
The advantages of the cable construction of this invention include a savings in cost, a streamlined economy of size and weight, and an improved flexibility, while also improving the operative frequency range and temperature capabilities of the cable.